Plants are harvested for a variety of useful products. Although some plants are coveted for their leaf such as spinach and lettuce, the stalk such as asparagus, and the root such as carrots, many plants are useful for some aspect of their reproductive cycle, such as the flowering portion (roses), the fruit such as avocado, and the seed such as corn and wheat.
In addition to water and carbon dioxide that plants require to grow, plants also require minerals to grow in a healthy manner, and to provide us with those nutrients that are essential to our own health. These minerals are normally absorbed through the roots, though in some cases foliar application is effective.
Although the specific needs and relative proportions of nutrients needed by different species of plants may vary, as a general rule, all plants require the same nutrients. The necessary proportions, however, vary from specie to specie, as well as throughout the life cycle of a plant. Environmental ranges of light, temperature, humidity, airflow, etc. can also have a controlling effect on ideal nutrient composition for various crops in their range. Optimizing nutrient delivery throughout the plant lifecycle may optimize the growth or development of those aspects of a plant found most desired.